Optical scanner devices are well-known in the art and may be used to produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by an optical scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer. A hand-held or portable optical scanner is an optical scanner which is designed to be moved by hand across the object or document being scanned. The hand-held scanner may be connected directly to a separate computer by a data cable. If so, the data signals produced by the hand-held scanner may be transferred to the separate computer "on the fly," i.e., as the image data are collected. Alternatively, the hand-scanner may include an on-board data storage system for storing the image data. The image data may then be downloaded to a separate computer after the scanning operation is complete by any convenient means, such as via a cable or an optical infrared data link.
A typical hand-held optical scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (commonly referred to as a "scan region"), whereas the optical system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan region (commonly referred to as a "scan line") onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the scan line across the entire object, usually by moving the hand-held scanner with respect to the object. By way of example, the illumination system may include a plurality of light emitting diodes (LEDs), although other types of light sources, such as fluorescent or incandescent lamps, may also be used. The optical system may include a "contact image sensor" or CIS to focus the image of the illuminated scan line onto the surface of the detector. Alternatively, a lens and/or mirror assembly may be used to collect and focus light from the illuminated scan region onto the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system typically comprises a charge-coupled device (CCD), although other devices may be used. A typical CCD may comprise an array of individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. Since the quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots of an image focused thereon.
The term "image light" as used herein refers to the light that is focused onto the surface of the detector array by the optical system. Depending on the type of scanner and the type of document, the image light may be reflected from the object being scanned or it may be transmitted through the object. The image light may be converted into digital signals in essentially three steps. First, each pixel in the CCD detector converts the light it receives into an electric charge. Second, the charges from the pixels are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital signals then may be processed and/or stored as desired.
The hand-held scanner device may be provided with a position sensing or "navigation" system in order to determine the position of the hand-held scanner with respect to the object being scanned. Accordingly, such a position sensing system allows the scanner to correlate its position with respect to the object being scanned. The position correlation allows a complete image of the scanned object to be produced even though the scanner may not scan the entire object during a single pass or "swipe." For example, if two or more swipes of the object are required to scan the entire object, then the position correlation provided by the navigation system will allow the various portions of the scanned image data to be "stitched" together to form a single unitary image representative of the entire scanned object.
One type of navigation system utilizes a pair of optical sensors to detect certain inherent structural features (e.g., surface roughness, paper fiber orientation, etc.) contained on the object being scanned (e.g., a sheet of paper with text or images thereon). Examples of the foregoing type of navigation system are disclosed in U.S. Pat. No. 5,089,712 of Holland for "Sheet Advancement Control System Detecting Fiber Pattern of Sheet," and U.S. Pat. No. 5,578,813 of Allen, et al., for "Freehand Image Scanning Device which Compensates for Non-Linear Movement," both of which are specifically incorporated herein by reference for all that they disclose.
One problem associated with many hand-held scanners relates to the ability of the user to keep track of the position of the scan line with respect to the body of the scanner. That is, since the aperture in the image head of the scanner is not visible to the user during scanning, it is often difficult for the user to remember precisely the location of the scan line. If the user's recollection is incorrect, he or she may fail to capture an image of the entire object or document being scanned. If so, the user will need to re-scan the entire object.